Migrating to exchange 2003

Scenario:

Unix send mail server in place.
Nt mixed mode domain with 4.0,2000, & 2003 servers
Roughly 1200 AD users accounts

Problem:

Usernames in send mail don?t coincide with usernames in AD

Question:

During the exchange install is there away that you can force all the newly created exchange email accounts to have email addresses formatted, such as firstname_lastname@wherever.com? If so, can you also still keep the existing user account login usernames the same?